fix(stl): keep the own normal of small facets

The vertex-normal fallback compared the cross product's length with
f32::EPSILON, an area-scale threshold, so a valid facet with 1e-4 edges
(cross product 1e-8) took the leaning vertex normal again. Fall back only
when the cross product is zero.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/io/stl.rs

@@ -40,18 +40,19 @@ pub fn build_stl(meshes: &[&MeshModel]) -> Option<Vec<u8>> {
             let c = verts[i2];
 
             // The facet normal is the triangle's own. A smoothed vertex normal
-            // leans off it, so only a triangle with no area falls back to the
-            // normal the mesh gives it.
+            // leans off it, so only a triangle whose cross product is zero
+            // falls back to the normal the mesh gives it. Small facets have a
+            // tiny cross product but still a normal of their own.
             let ab = [b[0] - a[0], b[1] - a[1], b[2] - a[2]];
             let ac = [c[0] - a[0], c[1] - a[1], c[2] - a[2]];
             let nx = ab[1] * ac[2] - ab[2] * ac[1];
             let ny = ab[2] * ac[0] - ab[0] * ac[2];
             let nz = ab[0] * ac[1] - ab[1] * ac[0];
             let len = (nx * nx + ny * ny + nz * nz).sqrt();
-            let normal = if len <= f32::EPSILON && i0 < mesh.normals.len() {
+            let normal = if len == 0.0 && i0 < mesh.normals.len() {
                 mesh.normals[i0]
             } else {
-                let len = len.max(f32::EPSILON);
+                let len = len.max(f32::MIN_POSITIVE);
                 [nx / len, ny / len, nz / len]
             };
 
@@ -141,6 +142,23 @@ mod tests {
         assert_eq!(first_facet_normal(&stl), [0.0, 0.0, 1.0]);
     }
 
+    /// A small facet still has a normal of its own. Its cross product is tiny
+    /// (1e-8 here), far below f32::EPSILON, but it is not zero.
+    #[test]
+    fn a_small_facet_keeps_its_own_normal() {
+        let leaning = [
+            std::f32::consts::FRAC_1_SQRT_2,
+            0.0,
+            std::f32::consts::FRAC_1_SQRT_2,
+        ];
+        let mesh = triangle(
+            vec![[0.0, 0.0, 0.0], [1e-4, 0.0, 0.0], [0.0, 1e-4, 0.0]],
+            vec![leaning; 3],
+        );
+        let stl = build_stl(&[&mesh]).expect("stl");
+        assert_eq!(first_facet_normal(&stl), [0.0, 0.0, 1.0]);
+    }
+
     /// A facet with no area has no normal of its own, so it keeps the one
     /// the mesh gives it rather than a zero vector.
     #[test]